1. Field
The present invention relates to techniques for charging batteries. More specifically, the present invention relates to a technique for charging a lithium rechargeable battery using multistep constant-current and constant-voltage charging steps.
2. Related Art
Rechargeable lithium batteries, such as lithium-ion or lithium-polymer batteries, are presently used to power a wide variety of portable electrical systems, including laptop computers, cell phones, PDAs, digital music players and cordless power tools. A significant challenge in charging lithium rechargeable batteries is to avoid lithium surface saturation at a transport-limiting electrode, while keeping the charging time to a minimum. For example, a profile of a conventional constant-current charging technique is illustrated in FIG. 1. As illustrated in FIG. 1, a single cell is charged at a 0.5 C rate until the cell voltage reaches 4.2V. Once the cell voltage reaches 4.2V, the cell is topped off using a constant-voltage, wherein the current is allowed to decay to a lower limit 0.05 C rate. (C is defined as a multiple expressed in amperes (A) of the rated cell capacity for the cell expressed in ampere-hours (A·hr). For example, for a cell with a capacity of Qmax=2500 mA·hr, a “1 C” current would be 2500 mA.)
Conventional charging techniques have a number of drawbacks, such as poor charging efficiency at low temperatures, which can lead to lithium plating on the anode surface. Moreover, in a conventional battery pack, the individual cells and cell banks are often organized in a series stack, and can become unbalanced due to inadequate charging, which can result in a shortened battery cycle life. In addition to a shortened cycle life, constant-current charging techniques can also cause unsafe conditions in portable systems resulting from internal short formation due to Li plating.
Hence, what is needed is a method and an apparatus for charging a rechargeable battery without the drawbacks of the above-described conventional charging techniques.